Production of red phosphorus by the thermal conversion reaction of yellow phosphorus is known. For example, a process in which yellow phosphorus is converted to red phosphorus by heating in a closed vessel is described in Kirk-Othmer "Encyclopedia of Chemical Technology" (2nd Ed.) Vol. 15, p. 286. In this process, after the reaction, the red phosphorus is chipped out under water, wet-ground, and sieved to give a red phosphorus powder.
In Industrial & Engineering Chemistry Vol. 40, P. 357 ('48), a continuous process is described. In this process, a slurry comprising a mixture of yellow phosphorus and red phosphorus is prepared by a thermal conversion reaction and the yellow phosphorus is separated from red phosphorus by evaporation in a screw conveyor type distilling device while the red phosphorus is powdered. Techniques related to this process are described in U.S. Pat. Nos. 2,476,335 and 2,397,951.
In U.S. Pat. No. 4,273,752 is described a process in which yellow phosphorus is introduced into a ball mill preheated at from 280.degree. to 590.degree. C. to convert the yellow phosphorus to red phosphorus and, after cooling, water is introduced into the ball mill to wet-grind the red phosphorus, which is then taken out as an aqueous suspension.
U.S. Pat. No. 4,526,766 discloses a process in which yellow phosphorus is converted to red phosphorus by the conversion heat thereof.
Red phosphorus is known as a flame retardant for synthetic resins. As is well known, red phosphorus decomposes by a disproportionation reaction in the presence of water to yield a substance which contaminates the environment or denatures synthetic resins. The decomposition reaction is accelerated at higher temperatures. Therefore, more stable and less reaction is needed especially for the resins molded at high temperatures. On the other hand, red phosphorus powder composed of fine particles is demanded in order to enhance compatibility with resins and dispersibility in resins. The prior-art techniques described above necessitate pulverizing of the red phosphorus by pulverizing process. However, since pulverizing forms split-off surfaces which are chemically unstable, water and other substances are apt to be adsorbed onto the surface of red phosphorus particles and the adsorbed substances accelerate the disproportionation reaction. The more finely the particles are reduced by pulverizing, the more the stability of the particles is impaired. Therefore, in the processes including pulverizing process, stability is incompatible with reduction into fine powder and it is impossible to obtain a stable red phosphorus fine powder.
In U.S. Pat. No. 4,879,067 (the same assignee of the present invention), red phosphorus in the form of spherical particles is described which has been obtained by a process including no pulverizing process. This red phosphorus composed of spherical particles is obtained by thermally converting yellow phosphorus at a temperature of from 250.degree. to 600.degree. C. in a conversion ratio not higher than 70%, and is highly stable because no pulverizing was used. However, in this process, the smaller the particles, the more the particle size control becomes difficult. The above process is hence unsuited for the production of fine powders of red phosphorus.